Physical Structure and older stratified Deposits of Devonshire. 651 



Ashburton limestone contains a few corals and some unascertained casts of shells. Immediately 

 under one of the coral beds of the Ashburton quarries, is a remarkable band of highly carbona- 

 ceous black shale. The neighbouring beds of limestone are of a dark colour and very much re- 

 semble true carboniferous limestone. They do not, however, burn (like the dark beds of moun- 

 tain limestone) into white lime, and therefore do not derive their chief colouring matter from 

 carbon*. 



Still higher in the ascending section, the calcareous bands disappear, and we have again a suc- 

 cession of soft glossy slates (in some places used for roofing), with very few distinctive mineral 

 characters, and generally without any cleavage, separate from the laminae of deposit. The upper 

 part of this series again becomes calcareous ; obscure casts of organic remains are found abun- 

 dantly in some parts of it ; and among indurated slates associated with trap, which seem to under- 

 lie the limestone of Newton Bushel, are fine impressions of an undescribed Trilobite. 



The fossiliferous slaty beds introduce the great upper or Torbay limestone, which is magnifi- 

 cently developed both in the interior of the country and on the coast. 



The nature of this communication not admitting of minute details, we have so far omitted all 

 description of the various trap rocks which occur on our lines of section. Some are of the nature 

 of dykes, and appear to have been protruded at different times prior to the eruption of the gra- 

 nite, which (as was pointed out by Mr. Austen) they never penetrate. Other trappean masses 

 are not only parallel to the beds of schist, but are associated with plutonic sediment passing into, 

 and alternating with, the slate rocks. In such cases, we concluded (as we had done before in num- 

 berless instances seen in Cumberland and Wales) that the plutonic and aqueous rocks were con- 

 temporaneous. 



In the slaty series between the Ashburton bands and the great upper limestone on the east side 

 of South Devon, we may often remark, that the higher portions become arenaceous, and sometimes 

 pass into a coarse red sandstone, not to be distinguished from the old red sandstone. Instances 

 of this kind are exposed in Babbacombe Bay, in the hills N.W. of Totness, and on the banks of 

 the Dart. Neither is this series altogether devoid of calcareous matter and beds of limestone, as may 

 be seen in several sections to the north-west of Totness. We considered these beds as mere offsets 

 from the great limestone ; but Mr. Austen has since shown us sections, which prove that at least 

 some of them are true members of the middle slate series. They cannot, we believe, be traced 

 far on the lines of strike, as many transverse sections give no indication of them. A more import- 

 ant fact, however, in the history of this great slaty group is the frequent occurrence of organic 

 remains in the beds as they approach the great upper limestone. This fact (of which examples 

 may be seen near Ogwell, near Pomeroy Castle, in the cliffs east of Torquay, at Mudstone Sands, 

 &c. &c.) shows that the slates and overlying limestone belong to one uninterrupted series of de- 

 posits ; for in other situations the slates are very rarely fossiliferous. 



The upper limestone in the eastern parts of South Devon generally ap- 

 pears in the form of great unconnected masses, more or less tabular, crowning 

 the hills and surmounted by no newer deposits. In its structure, as well as 



* Among the shales which divide the beds of the great upper limestone of South Devon, about 

 two miles north of Totness, we found a few thin laminas of bright coal. Examples of this kind 

 are, we believe, very rare ; but there are similar appearances in the Hooe limestone quarry, near 

 Plymouth. 



VOL. V. — SECOND SERIES. 4 P 



